1. Field of the Invention
This invention relates to assembled batteries and, more specifically, to layers and parts to insulate and protect metallic parts that electrically connect terminals of neighboring single cells in constructing assembled batteries.
2. Description of the Prior Art
While automotive application is one of the major applications of rechargeable batteries as represented by lead-acid batteries, various types of high energy density, small size and light weight rechargeable batteries have been developed in recent years, and are finding uses in many fields and the market is rapidly expanding. Typical examples of these applications include cordless equipment such as portable telephones, lap top computers, camcorders, and batteries for power supply such as uninterrupted power system (UPS) for computer backup and electric vehicles.
Thanks to advances in electronics technologies, cordless equipment consumes increasingly less power and the operating voltage and load current of the batteries for power supply have been suppressed. As a result, single cell or assembled batteries consisting utmost ten cells connected in series are often used as a battery pack.
On the other hand, with batteries intended for use as a power supply requiring operation at a high voltage and with heavy load as in stationary power supply for emergency and electric vehicles, a module battery with a desired nominal voltage is constructed by electrically connecting in series neighboring terminals of a plurality of single cells of the same nominal capacity by means of connecting bars, each single cell comprising positive electrodes and negative electrodes with a separator disposed between each of the positive and negative electrodes and housed in a battery container thereafter being injected with a volume of electrolyte. Depending on the requirement of nominal voltage, a plurality of such single cells are electrically connected in series and/or in parallel to make assembled batteries as a module battery.
In order to attain small size and light weight, module batteries usually use mono-block containers to allow integrated assembling of multiple single cells with the same capacity to obtain a nominal voltage of 6 volts or 12 volts by connecting them in series. However, in the case of a high nominal capacity battery exceeding 100 Ah, because of the constraint of available molding technology and economy of the battery container, module batteries and assembled batteries are usually made by assembling single cells housed in single-cell containers and connecting them in series and/or in parallel. For the connecting bar to electrically connect single cells, copper plate is generally used because of low ohmic resistance and relatively low cost. The connecting bar is fit to a pair of threaded bolt-like electrode poles that form the terminals of neighboring single cells, thereafter fastened by means of nuts together with washers to complete electrical connection. Surfaces of the metallic connecting parts including the connecting bar, electrode poles, washers, and nuts are generally plated with a corrosion resistant metal, such as lead (Pb) for lead-acid batteries, and mostly nickel (Ni) and in some cases silver (Ag) or gold (Au) for alkaline storage batteries such as of nickel-cadmium and nickel-metal hydride systems.
Among the batteries for power supply use, batteries for electric vehicles are required to have mechanical strength to withstand vibration, shock, acceleration, etc., which is equivalent to what is required of components of automobiles driven by internal combustion engines such as conventional gasoline engines and diesel engines. These batteries are also required to have not only high energy densities but also endurance to exhibit stable output characteristics over a wide temperature and humidity range under sustained exposure to dusts, splashes and corrosive substances over a long period. Even when the metallic parts that electrically connect individual single cells are plated with anticorrosive metal as described earlier, it is practically impossible to completely avoid pin holes in the plated metal layer, and sometimes corrosion appears on these metallic parts. Also, as these batteries are exposed to high temperature and high humidity over a long period, electrolyte leakage phenomenon may take place in which the electrolyte in a cell creeps up along a terminal and causes salting of the terminal and the vicinity. Since electric vehicles at times run on rough unpaved roads or winter roads sprinkled with antifreezing agents, there is a possibility that foreign objects get inside the installed housing of assembled batteries causing contamination and damage of the metallic parts and the battery surface. The foreign objects attached to and/or produced on the metallic parts and vicinity not only spoil the appearance but may also cause keeping of water resulting from moisture condensation due to temperature difference, or water splashes caused by car washing or rain, thereby wetting the metallic parts and the nearby top surface of the cover of each single cell ending in a liquid junction.
The total voltage of an electric vehicle battery ranges from about 100 volts to around 300 volts or in some case even to as high as 400 volts. When a high voltage is applied to a liquid junction formed as described above, a leakage current flows and causes reduction in the capacity as if some of the single cells in the assembled batteries have self-discharged. This further causes capacity fluctuation among individual single cells of the assembled batteries deteriorating the output characteristics of the electric vehicle battery. In an extreme case, it was observed that a single cell with reduced capacity had caused reversing polarity toward the end of discharge starting to evolve gas, and resulted in unrecoverable deterioration. Also, when the leakage current is large, there is a danger of fire due to heat generation or ignition. Therefore, it has been necessary to frequently clean and remove foreign objects from the surface of assembled batteries for electric vehicles, and the maintenance has been extremely troublesome.
FIG. 1 shows a partially cutaway perspective view of a single cell of nickel-metal hydride system that makes up a prismatic sealed alkaline storage battery for electric vehicles as an embodiment of this invention. In FIG. 1, an electrode group 5 is constructed with a plurality of positive plates and negative plates alternately stacked with a separator made of hydrophilic treated polypropylene nonwoven cloth disposed between each of the positive and negative plates. The positive plates are prepared by filling a high porosity nickel sheet with an active material paste mainly consisting of nickel hydroxide, followed by drying and pressing. The negative plates are prepared by coating both sides of a nickel plated and perforated steel sheet with a paste mainly consisting of hydrogen absorbing alloy powder of the MmNi.sub.5 (Mm: Misch metal) group, followed by drying and pressing. Lead plates 5a of the positive and negative plates are respectively connected and fixed by welding to the bottom part of the base 2a of a pair of the electrode poles 2. After fitting an O-shaped ring 3 on each of the electrode poles 2 and fixing on the top surface of the base 2a, a pair of electrode poles 2 are inserted through the terminal holes (not shown in the drawing) of a synthetic resin cover 1 composing mainly of polypropylene, thereafter pushnuts 4 are press-fit to the top surface of the cover 1 thereby press-deforming the O-shaped rings 3 underneath the bottom surface of the cover 1 in a liquidtight and airtight manner. The electrode group 5 fixed to the cover 1 is inserted and housed in a cell container 6 made of the same material as the cover 1. The lower edge of the cover 1 and the upper open edge of the cell container 6 are thermally or ultrasonically fused together in a liquidtight and airtight manner. Preparation of a single cell is completed by pouring a predetermined amount of alkaline electrolyte mainly composing of potassium hydroxide, impregnating the electrode group 5 therewith, and providing a safety valve 7 in an airtight manner.
In constructing a module battery of 6 volts or 12 volts, 5 or 10 single cells prepared as above and having the same nominal capacity are electrically connected in series by connecting neighboring terminals of single cells by means of connecting bars.
FIG. 2 shows a partially sectional view of an example of conventional method of electrically connecting in series the terminals of neighboring single cells. In FIG. 2, a connecting bar 8 made of nickel-plated copper plate is press-fit to the electrode poles 2 of opposing polarity that have been fastened by pushnuts 4 and projecting upwardly of the top surface of each of the covers 1, and fastened by screwing with nuts 2b together with washers 2c thus completing electrical connection. Conventionally, metallic parts for electrical connection and the top surface of the covers 1 were covered with a protective cover 12 made of synthetic resin or synthetic rubber. Though this protective cover 12 was effective in preventing short-circuit between the terminals on the top surface of the battery, it was not sufficiently airtight and could not perfectly shut off the outside air from the battery allowing entry and attachment of foreign objects such as dusts and splashes. Therefore, leakage current due to a liquid junction at the terminals and vicinity continued to be caused requiring periodic cleaning, and maintenance problems remained unsolved.